Certain steels require very consistent uniformity of hardness and uniformity of grain structure. Such steels often have chemistries which make the attainment of these properties exceedingly difficult by conventional air or liquid quench methods. For example, certain steels such as tool steels and other steels intended for specialized applications require high hardness and excellent mechanical properties. These requirements in turn dictate the presence of substantial amounts of alloying elements. However, the substantial alloy contents make attaining the requirements of high hardness and excellent mechanical properties difficult to obtain by conventional methods of manufacture. Thus, steels which have the chemistries corresponding to the attainment of high hardness often lack the attributes of retention of that hardness following a relatively low temperature stress relief treatment. The end result is less than desired mechanical properties.
As another example, air hardening tool steels, if water quenched, generally crack. As a consequence, chemistries of this type of steel require a very high quality of air quench in order to maximize hardness and mechanical properties. Thus, it will be noted that there are at least two types of steels that require the best air quench possible: (1) tool steels with significant alloy contents which tend to crack on water quenching; and (2) steel chemistries wherein the requirements of high hardness and desired mechanical properties dictate very low tempering temperatures (which can make the steel brittle). The brittle condition generally requires the use of stronger chemistries to permit higher tempering temperatures with the result that water quenching may have to be eliminated all together.
In addition to the special situations described above, the conventional mode of air quenching, including forced draft air quenching as that term is currently understood, does not always yield the desired results required in air quench steels intended for use in more generalized applications. Forced draft air quenching as that concept is currently practiced involves austenitizing steel pieces and then placing them on the shop floor, either in a pile of spread out, and thereafter permitting the pieces to self-cool or be subjected to a fan located nearby which blows a draft of air onto or across the pieces.
This technique has several drawbacks. One is the fact that adjacent pieces often slack quench one another which amounts, in essence, to an irregular quench pattern which in turn results in differing properties from location to location on the same piece of steel. This is believed attributable to the hot spots which occur at those discrete locations on separate pieces which are disposed closest and in direct juxtaposition to one another, and the further fact that scale which forms on the surface during quenching is retained in place, which scale further disturbs the regularity of the cooling pattern.
The result of these deficiencies of current methods is a large percentage of re-heat treatments, often referred to as retreats, and redraws. Indeed, a retreat rate of 1.8 times has been experienced in one shop which practices strict adherence to conventional standards.
A further concern of all specialty steel makers is the challenge presented by higher and higher standards for steel imposed by certain industries. By way of example, the die casting industry is turning more and more to the H13 Premium grade. Chrysler Automotive Corporation has developed a specification for an ultra-clean H13 steel. These steels, until now, were thought to require a double melting process, usually the vacuum arc remelt (VAR) process. Such double melting, while yielding excellent product, as contrasted to most single melting processes, is quite costly. A consistently fine grained, non-segregated product is produced, the latter characteristic being attributable to the rapid second cooling in the VAR process; i.e.: the solidification is so rapid that there is insufficient time for alloy segregation to take place, and hence a high percentage of the alloys are retained within the grain boundaries.
It has been found, however, that when the single melting process inherent in the vacuum arc degassing process is practiced with care, and this invention is practiced in conjunction therewith, the H13 Premium and Chrysler Automotive Corporation specifications can be approached, and will be met, without the need to resort to the more expensive and time consuming double melt VAR process. Indeed, the use of the present invention in conjunction with the vacuum arc degassing process has resulted in a steel which meets all H13 standards with the exception of the Type D inclusion rating, as to which the present invention has yielded a rating of 11/2 instead of the required 1 on a scale of 5.
This invention overcomes the above described problems and others, and produces steel parts having consistent uniformity of hardness and consistently desirable grain structures by the application of a forced draft of air, or other gaseous quench medium, to parts which are so arranged with respect to one another as to eliminate slack quenching.
In addition, the forced draft cooling of this invention results in less warpage than is experienced with drastic quenches while, at the same time, providing the above advantages. This is particularly important to the die industry which, up to now, has usually had to sink and then harden to be sure of perfectly flat surfaces in the final die.
The advantages of the present invention can be supplemented by use of a special liquid quench. Specifically, parts may be first liquid quenched to an interim temperature level, and thereafter immediately subjected to the forced air quench described herein to attain a slow, uniform temperature reduction which yields the desired martinsitic microstructure.
Accordingly, an object of this invention is to provide a method and apparatus for forced draft cooling of steel work pieces which results in a high degree of consistency of uniformity of hardness and consistency of grain structure.
Another object is to provide a method and apparatus of quenching entirely or significantly by forced air cooling which eliminates problems common to conventional still air or forced air methods, including such problems as slack quenching.
Another object is to provide a method and apparatus of forced air quenching of steel parts which will significantly contribute to the attainment of VAR quality steel using the single melting vacuum arc degassing process, thereby eliminating a second melting and the use of specialized equipment.
A further object is to provide a method and apparatus of air quenching which produces product in which alloy segregation and other deleterious attributes of single melt melting processes are eliminated or substantially reduced.
These and other objects and advantages of the invention will be apparent from the following exemplary description of a currently preferred mode of practicing the invention.